1. Field of the Invention
The present invention relates to galvanized steel sheets which are used in automobile bodies and which have significantly improved perforative corrosion resistance after electrocoating, without adverse effects on other properties.
2. Description of the Related Art
Galvanized steel sheets have been widely used in order to prevent decreased strength of automobile bodies over the long term in corrosive environments. For example, in Japan, zinc-nickel alloy coated steel sheets and zinc-iron alloy coated steel sheets have been typically used. Although the zinc-nickel alloy and the zinc-iron alloy ensure high corrosion resistance of the steel sheets, these alloys have some problems.
The zinc-nickel alloy coated steel sheet is produced by an electroplating process and results in high material costs due to the use of nickel which is expensive. Moreover, the nickel content must be restricted to a narrow range, such as 12.+-.1 percent by weight, making the production of the zinc-nickel alloy steel sheet difficult.
The zinc-iron alloy coated steel sheet may be produced by either an electroplating process or a hot dipping process. When the zinc-iron alloy coated steel sheet is produced by an electroplating process, the iron content in the zinc coating layer also must be controlled within an extremely narrow range. Since ferrous (Fe.sup.2+) ions in the plating solution are readily oxidized, the zinc-iron alloy coated steel sheet cannot be stably produced, resulting in increased production costs.
In most cases, the zinc-iron alloy coated steel sheet is produced by a hot dipping process. In this process, molten zinc is coated on surfaces of a steel sheet, and the steel sheet is maintained at a high temperature to promote alloying of the steel and zinc. In this process, however, the quality of the steel sheet significantly depends on the aluminum concentration in the molten zinc plating bath, and the temperature and the time of the alloying step. Thus, advanced technology is required for the production of a uniform coating layer, resulting in increased production costs.
As described above, all the zinc-based alloy coating processes are difficult and incur increased costs.
On the other hand, galvanized steel sheets including only zinc layers can be produced by either electroplating or hot dipping at low cost. However, galvanized steel sheets have not been significantly used in automobile bodies due to the inadequate corrosion resistance thereof. When the galvanized steel sheet is exposed to a corrosive environment for long periods, the steel sheet is readily perforated due to corrosion, and the strength of the body is adversely affected.
In the production of an automobile body, a steel sheet or a coated steel sheet is subjected to press working, a chemical conversion treatment, electrocoating, and spray coating. Perforations due to corrosion typically form at the bottom portions of doors, because the bottom portions are bent and water which enters from gaps at the window collects at the bottoms of the doors, promoting corrosion of the steel sheet.
Among the above treatments, the chemical conversion treatment and the electrocoating treat the bent bottom portion of the door. However, the subsequent spray coating does not reach the narrow bent bottom portion. Since an improvement in corrosion resistance due to the spray coating is not achieved, perforative corrosion resistance after the electrocoating is significantly important.
In order to improve corrosion resistance of the galvanized steel sheet under such circumstances, methods for forming a phosphate film containing magnesium on a zinc-based coating layer by a chemical conversion treatment (phosphate treatment) have been disclosed.
For example, Japanese Unexamined Patent Application Publication No. 1-312081 discloses a surface treated metallic material having a phosphate coating film containing 0.1 percent by weight or more of magnesium formed on an electrogalvanizing layer. This metallic material having a magnesium-containing phosphate coating film has reduced rust formation in salt spray tests, but exhibits inadequate perforative corrosion resistance in a combined cycling corrosion test in which the corrosion is very similar to the actual corrosion of an automobile body.
Japanese Unexamined Patent Application Publication No. 3-107469 discloses a material having a phosphate coating film containing 1 to 7 percent of magnesium formed on an electrogalvanized layer. This material also has reduced rust formation in salt spray tests, but exhibits inadequate perforative corrosion resistance in the combined cycling corrosion test.
Japanese Unexamined Patent Application Publication No. 7-138764 discloses a zinc-containing metal coated steel sheet in which a zinc phosphate composite film containing zinc and phosphorus in a weight ratio (zinc/phosphorus) of 2.504:1 to 3.166:1, and 0.06 to 9.0 percent by weight of at least one metal selected from iron, cobalt, nickel, calcium, magnesium, and manganese is formed on a zinc-containing metal coating layer. This coated steel sheet exhibits superior high-speed press workability in automobile production, but has poor corrosion resistance and inadequate perforative corrosion resistance.
In summary, the zinc-based alloy plating incurs increased cost, while the use of the inexpensive zinc plating in automobile bodies results in inadequate corrosion resistance. Various methods have been attempted in order to improve corrosion resistance of the zinc plating. Among these, the formation of a phosphate coating film containing a specific amount of magnesium does not adequately improve perforative corrosion resistance.